Light in a mid-infrared wavelength range (wavelength of 5 μm to 30 μm, for example) is important in the field of spectroscopic analysis. In recent years, a quantum cascade laser (QCL) has been popular as a high performance semiconductor light source operating in the wavelength range (refer to Patent Literatures 1 to 8 and Non-Patent Literatures 1 to 9, for example).
The QCL is a unipolar laser utilizing a subband level structure formed in a semiconductor well structure to generate light through intersubband electron transition. The QCL has a cascade connection of a plurality of quantum well light emitting layers each having a quantum well structure that form an active region, thereby achieving high efficient and high output performance. The cascade connection of the quantum well light emitting layers is implemented by alternately stacking the quantum well light emitting layer and an electron injection layer, using the injection layer for injecting an electron into a light emission upper level.